Wire bonders are used during semiconductor assembly and packaging for making electrical wire connections between electrical contact pads on a semiconductor chip and a substrate, or between electrical contact pads on different semiconductor chips. Wire is fed from a wire spool containing bonding wire to a bonding tool such as a capillary for performing wire bonding at the bonding tool. The most widely used wire materials are gold, copper and aluminum. The electrical contact pads may comprise metallized bond sites on the semiconductor chip and on the interconnection substrates.
A typical method used to bond or weld the wire to a connection pad is through a combination of heat, pressure and/or ultrasonic energy. It is a solid phase welding process, wherein the two metallic materials (the wire and the pad surface) are brought into intimate contact. Once the surfaces are in intimate contact, electron sharing or interdiffusion of atoms takes place, resulting in the formation of a wire bond. The bonding force can lead to material deformation, breaking up of a contamination layer and smoothing out of surface asperity, which can be enhanced by the application of ultrasonic energy. Heat can accelerate inter-atomic diffusion, thus forming the wire bond.
One type of wire bond formation uses a ball wire bond. The process involves melting a sphere of wire material on a length of wire held by a capillary, which is lowered and welded to a first bonding position. The capillary then draws out a loop and then connects the wire to a second bond position using a bond that is usually of a crescent shape, commonly called a wedge wire bond. Another ball is then reformed for a subsequent first ball wire bond. Currently, gold or copper ball bonding is the most widely used bonding technique. Its advantage is that once the ball wire bond is made on the connection pad of a device, the wire may be moved in any direction without stress on the wire, which greatly facilitates automatic wire bonding.
During ball bonding operations, the second bond is in the form of the wedge wire bond at the second bond position. After the wedge formation, the bond head will move upwards when the wire is still connected to the wedge wire bond such that a length of wire is paid out between the capillary and the wedge wire bond. The bond head will rise to a tail height position, whereat a wire clamp of the wire bonding apparatus will close as the bond head moves to an Electric Flame-Off (“EFO”) level. During the further upwards motion when the wire clamp is closed, the wire will break off at the wedge wire bond location, thereby forming a wire tail which extends freely out of the capillary. An EFO device will produce a spark to melt the wire tail to produce a molten ball, which is used for forming the next ball wire bond.
The wire tail has to be of a sufficient length in order to produce a proper molten ball for the next ball bond. However, the wire tail may be too short or be missing due to various reasons. For instance, if the bonding parameters for the wedge formation are not well optimized or the material being bonded is contaminated, the wire may break off from the wedge wire bond prematurely. The wire may then unexpectedly fly out from the wire path or the capillary itself. If there is a short tail or no tail at all, a molten ball cannot be properly produced to make the next ball bond. If the apparatus detects that a proper wire tail is not formed indicating an operational error, the machine will stop and manual intervention by a human operator is generally required such as to rethread the bonding wire through the capillary. Accordingly, such errors in the formation of the wire tail lead to unnecessary machine downtime and reduce productivity.